Date: Fri, 20 Feb 2015 16:37:12 -0600 From: Alan Cox <alc@rice.edu> To: John Baldwin <jhb@freebsd.org> Cc: Adrian Chadd <adrian@freebsd.org>, "K. Macy" <kmacy@freebsd.org>, Alan Cox <alc@freebsd.org>, John-Mark Gurney <jmg@funkthat.com>, Konstantin Belousov <kib@freebsd.org>, "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org> Subject: Re: getting NUMA into the tree (userland most interesting for me) Message-ID: <968C1AD7-D806-4E69-87E4-AB88A4C5AA70@rice.edu> In-Reply-To: <2069208.rjIe3PXOHb@ralph.baldwin.cx> References: <20150219041012.GJ1953@funkthat.com> <CAJ-Vmok4peyq95o7%2BT7EkEEVb2ZqU3Y0pd_9kTMyBrxuhvX05w@mail.gmail.com> <CAHM0Q_Po7zkXhsS6N75sbLY1b5GmHmKbBE7T4z6dQg3CGWAuYw@mail.gmail.com> <2069208.rjIe3PXOHb@ralph.baldwin.cx>
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On Feb 20, 2015, at 2:14 PM, John Baldwin <jhb@freebsd.org> wrote: > On Friday, February 20, 2015 12:17:09 AM K. Macy wrote: >>>>> Yes, I think we have a fair bit to do in the kernel before we are = in a >>>>> position to export anything truly useful to userland = unfortunately. The >>>>> last time I talked with Jeff about projects/numa (after the first = draft >>>>> of the wiki page) I came away with the impression that there might = be >>>>> some things we can pull out of that branch, but that it isn't = suitable >>>>> for merging upstream directly. Jeff noted that he and Alan had = gone >>>>> through several iterations of this already (I believe at least 3 >>>>> completely different policy designs) all of which had their own = issues. >>>>>=20 >>>>> Outside of the VM I think that we can keep the APIs somewhat = stable by >>>>> having this opaque policy cookie to pass around that we can = redefine >>>>> the guts of later. However, various parts of the VM all have to = handle >>>>> whatever the policy defines, and while the vm_phys bits and >>>>> contigmalloc() might be kind of obvious to implement, higher level = VM >>>>> layers like kmem() and malloc() are more complicated. One thing = that >>>>> is in projects/numa is changes for UMA that we can hopefully reuse = much >>>>> of, but I don't recall how much (if any) of kmem/malloc is in = there.=20 >>>>> Also, while vm_phys is one of the first things to do, I know that = Alan >>>>> and Jeff have pending patches to remove the cache queue (since it = is >>>>> far less useful than it seems) which simplify vm_phys making it = easier >>>>> to implement NUMA policies there, so I'm hoping we can get that in >>>>> sooner before having to start tearing up the VM too much. This is = why >>>>> the stuff I currently have is targeted non-VM bits like interrupts = as >>>>> getting that correct is lower-hanging fruit that might provide = some >>>>> gains regardless. Even once vm_phys is done I think the first = thing to >>>>> tackle next is contigmalloc to facilitate static bus_dma = allocations >>>>> (descriptor rings and such) being local to a device. >>>>=20 >>>> Contigmalloc improvements and cache queue removal are in the >>>> phabricator queue now. They are also prerequisites for per-cpu free >>>> page caches which are a huge scalability improvement for some >>>> workloads such as Netflix's. >>>>=20 >>>> There is still a fair amount of scalability work (including = Jeffr's >>>> per-domain pagedaemon work) that really needs to happens before we = can >>>> seriously think about a general user-level NUMA interface. >>>=20 >>> Is there anything wrong with maybe bringing over the basic low level >>> allocator changes from projects/numa so the basics are there? >>=20 >> I think they're probably predicated on the work that is being >> shepherded in now. Even if not, it would require someone to shepherd >> it in and the corresponding spare cycles from alc to review / revise = / >> repeat - which seem to be in short supply. >=20 > Can you add entries for these to the wiki page with links to the phab = reviews? =20 > I know there is an entry for the page cache queue removal already, but = you=20 > could add one for contigmalloc right next to it. >=20 Essentially, the =93Remove the =91cache=92 page queue=94 task has a = number of significant subtasks that aren=92t listed, and the = contigmalloc() rewrite is the biggest of them. Specifically, the = current contigmalloc(M_WAITOK) implementation exploits the existence of = the =91cache=92 page queue, and to eliminate that dependence requires = the M_WAITOK case to work very differently.
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